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I did turn on show_stacktrace but no additional information was generated.I'm trying the upgrade using the cli command: "php tools/upgrade.php upgrade".Perhaps the stacktrace option saves information to a specific file but I dont know where it could be.

Anyway, my information should be all you need to identify the problem.

The column review_round_id exists in two tables:1 - review_rounds: it's ok, all values are filled;

2 - review_assignments: it's not ok because this table was accepting not null values before 2.4.0 metadata definition!

There are at least two possible corrections:

a - remove the "notnull" restriction for the field review_round_id / table review_assignments from the metadata definition at reviews.xml

b - rewrite the upgrade script to fill information at the field before it turns on the not null restriction.

I cant say which one should be used because I dont know the impacts it will bring to the rest of the system.